get_pages fixes

[filosofo]

get_pages queries don't work as expected:

• meta_key and meta_value are listed as separate potential parameters, but they have to be used together, or you get really odd behavior. One should be able to use them separately, as in the following example, which should return an array of pages with the meta_key of 'whatever':

$pages = get_pages(array('meta_key' => 'whatever'));

The above fails fatally.

• LEFT JOIN is more efficient

[ryan]

(In [7746]) Fix meta_key and meta_value queries in get_pages(). Props filosofo. fixes #6773 for trunk